This invention is related to highly localized Infrared (IR) spectra on a sample surface utilizing an Atomic Force Microscope (AFM) and a variable wavelength modulated IR source, and specifically to a multiple frequency probe excitation technique leading to improved rejection of background effects and improved spatial resolution.
IR spectroscopy is a useful tool in many analytical fields, such as materials and life sciences. Conventional IR spectroscopy and microscopy, however, have resolution on the scale of many microns, limited by optical diffraction. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of the sample, such as location of different materials or molecular structures. Conventional infrared spectroscopy is a widely used technique to measure the characteristics of material. In many cases the unique signatures of IR spectra can be used to identify unknown material. Conventional IR spectroscopy is performed on bulk samples which gives compositional information but not structural information. Infrared microscopy allows collection of IR spectra with resolution on the scale of many microns resolution. Near-field scanning optical microscopy (NSOM) has been applied to some degree in infrared spectroscopy and imaging. Recently, a technique has been developed based on use of an AFM in a unique fashion to produce such localized spectra. This work was described in a publication entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” Optics Letters, Vo. 30, No. 18, Sept.5, 2005. This technique is also described in detail in U.S. Pat. Nos. 8,001,830 and 8,402,819, commonly owned by the assignee of this invention, and whose contents are incorporated by reference. Those skilled in the art will comprehend the details of the technique in the publication and patent applications. The general technique is also referred to as Photo-Thermal Induced Resonance, or PTIR. It has also been called photothermal AFM-IR.
PTIR has been demonstrated in both research laboratories around the world and in commercial instrumentation developed by the assignee of this application. Although the spatial resolution obtained using this technique for IR spectroscopic identification of sample composition is superior to more conventional techniques, certain spatial resolution factors have been identified that might limit the technique for some applications. Specifically, the heating-produced interaction between the sample and the probe tip is not limited to the area directly under the apex of the probe tip. Both sample heating in the area around the tip and heating of the air under the cantilever contribute to the measured effect on the probe and act to delocalize the probe-sample interaction. Since the PTIR technique typically relies on a pulsed IR source, these background effects correlate with the pulse frequency, which in current PTIR set-ups also correlates to the data acquisition window.
Wickramasinghe (Image Force Microscopy of Molecular Resonance: A Microscopic Principle, Wikramasinghe et al, Applied Physics Letters 97, 073121, 2010), incorporated herein by reference, has recently showed the ability to obtain optical spectroscopic information in the visual spectrum with the tip of an AFM by using a heterodyne detection technique. Specifically, the cantilever was oscillated at a frequency f1 close to the fundamental resonance of the cantilever fc1. Visible laser light sources were modulated at a second frequency fm such that f1+fm=fc2, where fc2 is the second resonance mode of the cantilever. Effectively, this approach stimulates a non-linear mixing effect on the probe at fc2. Wickramasinge attributed the nonlinear mixing to nonlinear tip-sample forces when the tip is very close to the sample in attractive “tapping” mode. Using this technique Wickramasinghe detected single molecules of a dye with strong absorption in the visible. Wickramasinghe also suggested that it should be possible to make similar measurements at other radiation frequencies, including infrared, but he did not address any specific practical implementations. For the IR case, the probe amplitude waveform will be a mix of the oscillation amplitude and the ringdown waveform due to IR pulses at frequency fm, which may be from absorption or other effects such as mentioned by Wickramasinghe. Wickramasinghe in his visual spectrum setup selects fm to be such that the sum of fm and fc1 is equal to fc2 a second harmonic of the cantilever, and since fc2 is a resonance, any interaction will be magnified and more easily detectable. Since the mixing effect is a sum/difference nonlinear mixing taking place only when the tip is very near the sample, i.e. at the bottom of the “tapping” (intermittent contact or non-contact) oscillation, data taken at the sum or difference frequency, (fc1−fm) and (fc1+fm)=fc2 only includes interaction of the tip itself with the surface,. This has the effect that measurements made at fc2 are not sensitive to effects that only have a component at fm, the radiation modulation frequency, thereby rejecting the background effects which happen at fm and only including effects which have an fc1 component, specifically the oscillating contact of the tip and sample. This has the effect of localizing the measurement.
In practice, Wickramasinghe's suggestion that the technique could be used in the IR is far from straightforward. Wickramasinghe used narrow band CW laser sources and the spectra he obtained only contained a few spectral data points, each requiring the use of a different laser. There are few broadly tunable sources in the infrared and even fewer that operate in CW mode. Even with such a source, Wickramasinghe teaches a technique that would require modulation of the light source at the difference frequency between a cantilever's first and second resonance. As will be described below, this is impractical in the IR. Cantilevers used in AC mode AFM typically have resonance frequencies in the range of 50 kHz to around 1 MHz or higher. The 2nd flexural resonant mode for a rectangular cantilever is roughly 6× higher in frequency, ranging then from 300 kHz to 6 MHz or higher. So an IR laser modulation frequency designed to excite fc2 needs to be in the range of 250 kHz to 5 MHz or more. The exact frequencies, however, are variable, depending on the exact dimensions and mechanical properties of the cantilever and a variation in manufacturing of 20-50% from nominal values is not uncommon.
A variety of modulation techniques exist, including photoelastic modulators, electro-optic modulators, acousto-optic modulators, mechanical choppers, piezo electrically actuated mirrors, etc, but each of these suffer from one or more limitations. Some modulators work at a limited wavelength range thus limiting the spectroscopic range available and some techniques operate only over a limited range of modulation frequencies. Thus, successful application of Wickramasinghe's technique in the mid IR would ideally require:                A broad wavelength IR source, e.g. tunable from 2.5 to 10 um or preferably 2.5 to 16 um.        An IR modulator operable over the entire mid IR wavelength range.        The ability to tune the frequency of the IR modulator over the range of difference frequencies desired, e.g. from 250 kHz to 6 MHz or more.The current inventors are not aware of generally applicable modulation techniques that can be tuned from 250 kHz to 6 MHz and are operable in the mid-IR from 2.5-16 um. Thus, it is the object of this invention to provide novel techniques practical in the IR to apply the heterodyne detection technique to highly localized AFM based IR spectroscopy.        